docs: def relation_to_role_and_interface(relation_name):
      """
      Given the name of a relation, return the role and the name of the interface
      that relation uses (where role is one of ``provides``, ``requires``, or ``peers``).
  
      :returns: A tuple containing ``(role, interface)``, or ``(None, None)``.
      """
      _metadata = metadata()
      for role in ('provides', 'requires', 'peers'):
          interface = _metadata.get(role, {}).get(relation_name, {}).get('interface')
          if interface:
              return role, interface
      return None, None